1. Field of the Invention
This invention relates to the recovery of silver from photographic films. More particularly, it relates to the recovery of silver from polyester photographic film by subjecting small pieces of the film to controlled conditions of high shear to remove the light-sensitive silver emulsion.
2. Description of the Prior Art
Polyethylene terephthalate (PET), or polyester as it is commonly known, has been widely used as a base material for photographic films. The PET film base is coated with an adhesive polymer layer of a polyvinylidene chloride (PVCD) based resin commonly referred to as saran. This polymer layer is needed to permit the adhesion of the light-sensitive emulsion layer, usually a silver halide dispersed in gelatin, to the PET base since the gelatin does not bond well to a clean polyester surface.
Each year, millions of pounds of X-ray and other photographic films are scrapped. It is obviously quite desirable to recover the valuable silver contained in this scrap film. It would also be desirable to recover the polyester base material in a pure form. Recovery of the silver and polyester in forms which are reusable reduces solid waste disposal problems and conserves our natural resources through recycling of these materials.
Prior art methods for recovering silver and/or the PET base material from photographic film have included treating the scrap material with hot caustic alkali solutions and with solvents. U.S. Pat. No. 3,652,466, which issued Mar. 28, 1972, to E. R. Hittel and assigned to the same assignee as the present application, is directed to a polyester recovery process which involves treating small pieces of scrap film with a caustic aqueous alkali solution to form a slurry. The slurry is then fed down into a classification column wherein the countercurrent flow of a slowly rising aqueous liquid tends to separate both the silver halide emulsion layer and the PVCD coating from the polyester pieces. The silver compounds are subsequently recovered from the sludge containing the emulsion and PVCD resin.
In U.S. Pat. No. 3,873,314, which issued Mar. 25. 1984, to J. T. K. Woo et al, scrap photographic film is treated by solvents such as dimethylformamide (DMF) and tetrahydrofuran (THF). The mixtures are subjected to low shearing action by stirring to dissolve the halide emulsion, and PVCD resin layer. In both U.S. Pat. Nos. 3,503,904, which issued to H. J. Dietz et al on Mar. 31, 1970, and 3,928,253, which issued to J. S. Thornton et al on Dec. 23, 1975, the film scrap is treated with an alkaline glycol such as ethylene glycol to remove the PVCD subcoating and the photographic emulsion layers. As in the case of the foregoing prior art patents, the mixtures are agitated through low shearing action.
It is also known that the silver halide emulsion can be readily stripped from PVCD coating on the polyester film simply by subjecting the film to hot water. The principal drawback to this and other prior art silver recovery techniques is that they are not efficient. A relatively significant amount of the silver is not recovered.
Also, where the emulsion and PVCD layer are removed simultaneously, subsequent separation to recover the silver is made more complicated. Moreover, merely subjecting small pieces of scrap film to a hot caustic or hot water or a solvent, and stirring the mixture in the manner described by the prior art causes the pieces to stick together when they contact one another so that a high degree of agglomeration occurs. This makes it extremely difficult to remove all the silver emulsion. It also makes it difficult to clean the individual polyester pieces sufficiently to permit recovery of commercially pure polyester.
The present invention overcomes the limitations and disadvantages of the prior art by providing a recovery process by which 99 percent or more of the silver can be recovered through the effective use of high shear conditions. Pure polyester and indeed other plastic base materials may also be recovered efficiently and economically by subsequently subjecting the scrap film to similar high shear conditions as described and claimed in allowed copending application Ser. No. 693,253, entitled "Recovery of Polyester Film Scrap by High Shear Conditions", filed the same date and by the same applicants as the present application.